Separation of compositions containing water and organic oxygenates

ABSTRACT

Water is separated from aqueous mixtures of organic oxygenates such as isopropanol by pervaporation through a non-porous separating membrane of a blend of polyvinyl alcohol and a polyacrylic acid mounted on a polysulfone porous support layer.

RELATED APPLICATIONS

Application Ser. No. 07/214,987 filed July 5, 1988, of MordechaiPasternak, Craig R. Bartels, and John Reale, Jr. and now pending isdirected to the separation of water from a hydrocarbon mixture with anorganic oxygenate by the use of membrane technology.

FIELD OF THE INVENTION

This invention relates to the dehydration of organic oxygenates such asalcohols. More particularly it relates to a membrane technique foreffecting separation of water from an aqueous mixture containingalcohols such as isopropyl alcohol.

BACKGROUND OF THE INVENTION

As well known to those skilled in the art, it is possible to removewater from mixtures thereof with organic liquids by various techniquesincluding adsorption or distillation. These conventional processes,particularly distillation, are however, characterized by high capitalcost. In the case of distillation for example the process requiresexpensive distillation towers, heaters, heat exchangers (reboilers,condensers, etc.), together with a substantial amount of auxiliaryequipment typified by pumps, collection vessels, vacuum generatingequipment, etc.

Such operations are characterized by high operating costs principallycosts of heating and cooling--plus pumping, etc.

Furthermore the properties of the materials being separated, as isevidenced by the distillation curves, may be such that a large number ofplates may be required, etc. When the material forms an azeotrope withwater, additional problems may be present which for example, may requirethat separation be effected in a series of steps (e.g. as in two towers)or by addition of extraneous materials to the system.

There are also comparable problems which are unique to adsorptionsystems.

It has been found to be possible to utilize membrane systems to separatemixtures of miscible liquids by pervaporation. In this process, thecharge liquid is brought into contact with a membrane film; and onecomponent of the charge liquid preferentially permeates the membrane.The permeate is then removed as a vapor from the downstream side of thefilm--typically by sweeping with a carrier gas or by reducing thepressure below the vapor pressure of the permeating species.

Illustrative membranes which have been employed in prior art techniquesinclude those set forth in the following table:

                  TABLE                                                           ______________________________________                                        Separating Layer   References                                                 ______________________________________                                        Nafion brand of    Cabasso and Liu                                            perfluorosulfonic acid                                                                           J. Memb. Sci. 24,                                                             101 (1985)                                                 Sulfonated polyalkylene                                                                          U.S. Pat. No. 4,728,429 to                                                    Cabasso et al                                              Sulfonated polyethylene                                                                          Cabasso, Korngold                                                             & Liu J. Pol. Sc:                                                             Letters, 23, 57                                                               (1985)                                                     Fluorinated polyether                                                                            U.S. Pat. No. 4,526,948                                    or Carboxylic Acid fluorides                                                                     to Dupont as                                                                  assignee of Resnickto                                      Selemion AMV       Wentzlaff                                                  brand of Asahi Glass                                                                             Boddeker &                                                 cross-linked styrene                                                                             Hattanbach                                                 butadiene (with quaternary                                                                       J. Memb. Sci. 22,333                                       ammonium residues on a                                                                           (1985)                                                     polyvinyl chloride backing)                                                   Cellulose triacetate                                                                             Wentzlaff, Boddeker                                                           & Hattanback, J.                                                              Memb. Sci. 22, 333 (1985)                                  Polyacrylonitrile  Neel, Aptel &                                                                 Clement Desalination                                                          53, 297 (1985)                                             Crosslinked        Eur. Patent 0 096                                          Polyvinyl Alcohol  339 to GFT as                                                                 assignee of Bruschke                                       Poly(maleimide-    Yoshikawa et al                                            acrylonitrile)     J. Pol. Sci.,                                                                 22,2159 (1984)                                             Dextrine -         Chem. Econ. Eng.                                           isophoronediisocyanate                                                                           Rev., 17, 34 (1985)                                        ______________________________________                                    

The cost effectiveness of a membrane is determined by the selectivityand productivity. Of the membranes commercially available, anillustrative membrane of high performance is that disclosed in EuropeanPat. No. 0 096 339 A2 of GFT as assignee of Bruschke--published 21December 1983.

European Pat. No. 0 096 339 A2 to GFT as assignee of Bruschke discloses,as cross-linking agents, diacids (typified by maleic acid or fumaricacid); dihalogen compounds (typified by dichloroacetone or1,3-dichloroisopropanol); aldehydes, including dialdehydes, typified byformaldehyde. These membranes are said to be particularly effective fordehydration of aqueous solutions of ethanol or isopropanol.

This reference discloses separation of water from alcohols, ethers,ketones, aldehydes, or acids by use of composite membranes. Specificallythe composite includes (i) a backing typically about 120 microns inthickness, on which is posirioned (ii) a microporous support layer of apolysulfone or a polyacrylonitrile of about 50 microns thickness, onwhich is positioned (iii) a separating layer of cross-linked polyvinylalcohol about 2 microns in thickness.

Polyvinyl alcohol may be cross-linked by use of difunctional agentswhich react with the hydroxyl group of the polyvinyl alcohol. Typicalcross-linking agent may include dialdehydes (which yield acetallinkages), diacids or diacid halides (which yield ester linkages),diahalogen compounds or epichlorhydrin (which yield ether linkages)olefinic aldehydes (which yield ether/acetal linkages), boric acid(which yields boric ester linkages), sulfonamidoaldehydes, etc.

See also J. G. Prichard, Polyvinyl Alcohol, Basic Properties and Uses,Gordon and Breach Science Publishers, New York (1980) or

C. A. Finch, Polyvinyl Alcohol, Properties and Applications, John Wileyand Sons, New York (1973).

T. Q. Nguyen et al Synthesis of Membranes for the Dehydration ofWater-acetic Acid Mixtures by Pervaporation Makromol. Chem 188,1973--1984 (1987).

H. Karakane et al Separation of Water-Ethanol by Pervaporation ThroughPolyelectrolyte Complex Composite Membrane. Proc. Third Int. Cont. onPervaporation Processes in the Chemical Industry, Nancy, France Sep19-22, 1988.

U.S. Pat. No. 4,755,299 to Bruschke, U.S. Pat. No. 4,802,988 to Bartelsand Reale, Jr., U.S. Pat. No. 4,728,429 to Cabasso et al, U.S. Pat. No.4,067,805 to Chiang et al, U.S. Pat. No. 4,526,948 to Resnick, U.S. Pat.No. 3,750,735 to Chiang et al, and U.S. Pat. No. 4,690,766 to Linder etal provide additional background.

It is an object of this invention to provide a novel composite membranecharacterized by its ability to effect separation of water from organicoxygenates such as isopropyl alcohol. Other objects will be apparent tothose skilled in the art.

STATEMENT OF THE INVENTION

In accordance with certain of its aspects, this invention is directed toa method of separating a charge aqueous composition containing organicoxygenate selected from the group consisting of alcohols, glycols, andweak acids which comprises

maintaining a non-porous membrane separating layer of a blend of apolyvinyl alcohol and a polyacrylic acid mounted on a polysulfone poroussupport layer;

maintaining a pressure drop across said non-porous membrane separatinglayer;

passing an aqueous charge composition containing water and organicoxygenate selected from the group consisting of alcohols, glycols, andweak acids into contact with the high pressure side of said non-porousseparating layer whereby at least a portion of said water in saidaqueous charge mixture and a lesser portion of organic oxygenate pass bypervaporation through said non-porous separating layer as a lean mixturecontaining more water and less organic oxygenate selected from the groupconsisting of alcohols, glycols, and weak acids than are present in saidaqueous charge and said charge is converted to a rich liquid containingless water and more organic oxygenate selected from the group consistingof alcohols, glycols, and weak acids than are present in said aqueouscharge;

recovering from the low pressure side of said non-porous separatinglayer said lean mixture containing more water and less organic oxygenateselected from the group consisting of alcohols, glycols, and weak acidsthan are present in said aqueous charge, said lean mixture beingrecovered in vapor phase at a pressure below the vapor pressure thereof;and

recovering from the high pressure side of said non-porous separatinglayer said rich liquid containing a lower water content and more organicoxygenate selected from the group consisting of alcohols, glycols, andweak acids than are present in said charge.

DESCRIPTION OF THE INVENTION

The composite structure of this invention includes a multi-layerassembly which in the preferred embodiment preferably includes a porouscarrier layer which provides mechanical strength and support to theassembly.

THE CARRIER LAYER

This carrier layer, when used, is characterized by its high degree ofporosity and mechanical strength. It may be fibrous or non-fibrous,woven or non-woven. In the preferred embodiment, the carrier layer maybe a porous, flexible, non-woven fibrous polyester.

A preferred non-woven polyester carrier layer may be formulated ofnon-woven, thermally-bonded strands and characterized by a fabric weightof 80±8 grams per square yard, a thickness of 4.2±0.5 mils, a tensilestrength (in the machine direction) of 31 psi and (in cross direction)of 10 psi, and a Frazier air permeability of 6 cuft/min/sq. ft. @0.5inches of water.

THE POROUS SUPPORT LAYER

The porous support layer which may be used in practice of this inventionis (preferably) formed of a sheet of polysulfone polymer. Typically thepolysulfone may be of of 40-80 microns, say 50 microns and of molecularweight M_(n) of 5,000-100,000, preferably 20,000-60,000 say 40,000. Thepolysulfone is preferably characterized by a pore size of less thanabout 500Å and typically about 200Å. This corresponds to a molecularweight cut-off of less than about 25,000 typically about 20,000.

The sulfone polymers which may be employed may include those made fromcumene, containing isopropylidene groups in the backbone; e.g. ##STR1##

These isopropylidene sulfones containing repeating units includingether-aromatic-isopropylidene-aromatic-ether-aromatic-sulfone-aromaticgroups may typically have a molecular weight M_(n) of 15,000-30,000, awater absorption (at 20° C.) of about 0.85 w %, a glass transitiontemperature of 449° K., a density of 1.25 g/cm³, a tensile strength (at20° C.) at yield of 10,000 psi, and a coefficient of linear thermalexpansion of 2.6×10⁻⁵ mm/mm/°C.

It is found, however, that the preferred sulfone polymers which may beemployed in practice of the process of this invention, may include thosewhich are free of isopropylidene moieties in the backbone chain andwherein the phenylene groups in the backbone are bonded only to etheroxygen atoms and to sulfur atoms. One preferred polymer, which maytypically, be prepared from ##STR2## may be characterized by a backbonecontaining the following repeating groups: ##STR3##

A preferred sulfone polymer may be a polyether sulfone which is free ofisopropylidene moieties in the backbone chain and wherein the phenylenegroups in the backbone are bonded only to ether-oxygen atoms and tosulfur atoms. This polymer may be characterized by molecular weightM_(n) of 25,000, water absorption @20° C. of 2.1 w %, glass transitiontemperature of 487° K., tensile strength at yield of 12,200 psig at 20°C.; and coefficient of linear thermal expansion of 5.5×10⁻⁵ mm/mm/°C.This polymer has a molecular weight cut off of about 20,000 and has apore size of about 200Å.

THE SEPARATING LAYER

In accordance with certain of its aspects, the separating layer may be ablend or mixture of vinyl alcohol polymer and a polymer of an acrylicacid such as acrylic acid or methacrylic acid. The charge from whichthis separating membrane may be prepared may be an aqueous solutioncontaining a vinyl alcohol polymer and a polymer of an acrylic acid.Typically the aqueous solution may contain 5-10 w %, say 7 w % ofpolyvinyl alcohol of molecular weight M_(n) of 20,000,-200,000, say115,000 and 5-10 w %, say 7 w % of polyacrylic acid of molecular weightM_(n) of 90,000-300,000, say 250,000. The weight ratio of vinyl alcoholpolymer to acrylic acid polymer may be 0.1-10:1, say 1:1. Generallydesirably higher Flux is attained by use of lower ratios e.g. 0.1-0.5,say 0.25.

When the separating layer is prepared from a mixture of vinyl alcoholpolymer and acrylic acid polymer (as in a preferred embodiment) it isdesirable to mix the aqueous solutions of polymers to form a mixcontaining both polymers.

The composite membrane, prepared from the blend of polyvinyl alcohol andpolyacrylic acid, may then be cured in an oven at 125° C.-225° C.,preferably 150° C.-225° C., say 150° C. for 1-30 minutes, say 10 minutesto yield a membrane of polyvinyl alcohol-polyacrylic acid film having athickness of 1-10 microns, say 2 microns. During heating for the notedtime, it appears that the components of the membrane system react orinteract to internally cure or cross-link the system; and no externalcuring agent is needed. In fact, presence of external curing agentsdenigrates against performance of the membranes.

During curing, the polyvinyl alcohol and the polyacrylic acid maycrosslink or otherwise react to form ester linkages. It also appearsthat the separating layer may interact with the polysulfone supportlayer to form a system characterized by unexpectedly high flux.

Illustrative polyvinyl alcohol-polyacrylic acid membranes which may beemployed may include:

TABLE

I. The membrane prepared by casting a mixture of equal parts by weightof a 7 w % solution of polyvinyl alcohol of M_(n) of 115,000 and a 7 w %solution of polyacrylic acid of M_(n) of 250,000, the mixture aftercasting being cured at 150° C. for 10 minutes to yield a film of about 2microns thick.

II. The membrane prepared by mixing equal parts of a 7 w % aqueoussuspension of polyvinyl alcohol of M_(n) of 115,000 and 7 w % aqueoussuspension of polyacrylic acid of M_(n) of 250,000 and casting themixture, followed by curing at 140° C. for 15 minutes to form a film ofthickness about 2.5 microns.

III. The membrane prepared by mixing equal parts of a 6 w % aqueoussuspension of polyvinyl alcohol of M_(n) of 100,000 and a 7 w % aqueoussuspension of polymethacrylic acid of M_(n) of 280,000 and casting themixture followed by curing at 150° C. for 10 minutes to yield a film ofthickness of about 2 microns.

THE COMPOSITE MEMBRANE

It is a feature of this invention that the composite membrane of thisinvention may comprise (i) an optional carrier layer, characterized byporosity and mechanical strength, for supporting a porous support layerand a separating layer, (ii) a polysulfone porous support layer ofmolecular weight cutoff of 20,000-40,000 and (iii) mounted thereon as anon-porous separating layer a blend of polyvinyl alcohol of molecularweight 20,000-200,000 and, say 115,000 and polyacrylic acid of molecularweight 50,000-350,000, say 250,000.

The composite membrane of this invention may be utilized in variousconfigurations. It is, for example, possible to utilize the composite ina plate-and-frame configuration in which separating layers may bemounted on the porous support layer with the carrier layer.

It is possible to utilize a spiral wound module which includes anon-porous separating layer membrane mounted on a porous support layerand carrier layer, the assembly being typically folded and bonded orsealed along all the edges but an open edge--to form a bag-like unitwhich preferably has the separating layer on the outside. A clothspacer, serving as the membrane or discharge channel is placed withinthe bag-like unit. The discharge channel projects from the open end ofthe unit.

There then placed on one face of the bag-like unit, adjacent to theseparating layer, and coterminous therewith, a feed channelsheet--typically formed of a plastic net.

The so-formed assembly is wrapped around a preferably cylindricalconduit which bears a plurality of perforations in the wall--preferablyin a linear array which is as long as the width of the bag-like unit.The projecting portion of the discharge channel of the bag-like unit isplaced over the perforations of the conduit; and the bag-like unit iswrapped around the conduit to form a spiral wound configuration.

It will be apparent that, although only one feed channel is present, thesingle feed channel in the wound assembly will be adjacent to two facesof the membrane layer. The spiral wound configuration may be formed bywrapping the assembly around the conduit a plurality of times to form areadily handleable unit. The unit is fitted within a shell (in mannercomparable to a shell-and-tube heat exchanger) provided with an inlet atone end and an outlet at the other. A baffle-like seal between the innersurface of the shell and the outer surface of the spiral-wound inputprevents fluid from bypassing the operative membrane system and insuresthat fluid enters the system principally at one end. The permeate passesfrom the feed channel, into contact with the separating layer and thencetherethrough, into the permeate channel and thence there-along to andthrough the perforations in the conduit through which it is withdrawn asnet permeate.

In use of the spiral wound membrane, charge liquid is permitted to passthrough the plastic net which serves as a feed channel and thence intocontact with the non-porous separating membranes. The liquid which doesnot pass through the membranes is withdrawn as retentate. The liquid orvapor which permeates the membrane passes into the volume occupied bythe permeate spacer and through this permeate channel to theperforations in the cylindrical conduit through which it is withdrawnfrom the system. In this embodiment, it will be apparent that the systemmay not include a carrier layer.

In another embodiment, it is possible to utilize the system of thisinvention as a tubular or hollow fibre. In this embodiment, thepolysulfone porous support layer may be extruded as a fine tube with awall thickness of typically 0.001-0.1 mm. The extruded tubes are passedthrough a bath of polyvinyl alcohol/polysulfone which is cured in situ.A bundle of these tubes is secured (with an epoxy adhesive) at each endin a header; and the fibres are cut so that they are flush with the endsof the header. This tube bundle is mounted within a shell in a typicalshell-and-tube assembly.

In operation, the charge liquid is admitted to the tube side and passesthrough the inside of the tubes and exits as retentate. During passagethrough the tubes, permeate passes through the non-porous separatinglayer and permeate is collected in the shell side.

In this embodiment, it will be apparent that the system may not normallyinclude a carrier layer. In still another embodiment, the porous supportlayer may be omitted; and the separating layer is extruded andthereafter crosslinked and cured in situ prior to mounting in theheaders.

PERVAPORATION

It is a feature of the non-porous polyvinyl alcohol--polyacrylic acidseparating layer on polysulfone support that it is found to beparticularly effective when used in a pervaporation process. Inpervaporation, a charge liquid containing a more permeable and a lesspermeable component is maintained in contact with a non-porousseparating layer; and a pressure drop is maintained across that layer.The charge liquid dissolves into the membrane and diffuses therethrough.The permeate which passes through the membrane and exits as a vapor maybe recovered by condensing at low temperature or alternatively may beswept away by use of a moving stream of gas. Preferably, the permeateside of the membrane is maintained at a low pressure, typically 5 mm.Hg.

For general background on pervaporation, note U.S. Pat. No. 4,277,344;U.S. Pat. No. 4,039,440; U.S. Pat. No. 3,926,798; U.S. Pat. No.3,950,247; U.S. Pat. No. 4,035,291; etc.

It is a feature of the process of this invention that the novel membranemay be particularly useful in pervaporation processes for dewateringaqueous mixtures of organic oxygenates selected from the groupconsisting of alcohols, glycols, and weak acids. It will be apparent tothose skilled in the art that it may be desirable to separate largequantities of water from partially miscible systems as by decantationprior to utilizing the process of the invention to remove the lasttraces of water.

The advantages of the instant invention are more apparent when thecharge liquid is a single phase homogenous aqueous solution as is thecase for example with isopropanol. It is also a feature of thisinvention that it may be particularly useful to separate azeotropes suchas isopropanol-water.

The charge organic oxygenates which may be treated by the process ofthis invention may include alcohols, glycols, and weak acids. It will beapparent to those skilled in the art that the charge organic oxygenatesused should be inert with respect to the separating membrane. Clearly asystem wherein the membrane is attacked by the components of the chargeliquid will not yield significant separation for any reasonable periodof time. Best results may be achieved when treating alcohols (such asisopropanol) or glycols (such as ethylene glycol). Results achieved withacids are generally less satisfactory.

Illustrative alcohols may include ethanol, propanol, n-butanol,i-butanol, t-butanol, amyl alcohols, hexyl alcohols, etc.

Illustrative glycols may include ethylene glycol, propylene glycols,butylene glycol or glycol ethers such as diethylene glycol, triethyleneglycol, or triols, including glycerine; etc.

Illustrative weak acids may include formic acid, oxalic acid, aceticacid, propionic acid, etc.

It is believed that the advantages of this invention are most apparentwhere the organic oxygenate is a liquid such as isopropanol which inpreparation or in use may pick up quantities of water from varioussources.

A typical charge may be an aqueous mixture containing 70%-99%, say about95% isopropanol.

In practice of the pervaporation process of this invention, the chargeaqueous organic oxygenate aqueous solution typically at 40° C-90° C.,say 65° C. may be passed into contact with the non-porous separatinglayer of the membrane of this invention. A pressure drop of about oneatmosphere is commonly maintained across the membrane. Typically, thefeed or charge side of the membrane is at about atmospheric pressure andthe permeate or discharge side of the membrane is at a pressure of about2-50 preferably 5-20, say 5 mm. Hg.

The permeate which passes through the membrane includes water and asmall proportion of the organic oxygenate form the charge liquid.Typically, the permeate contains 95-99, say 98 w % water. Permeate isrecovered in vapor phase.

Pervaporation may typically be carried out at a Flux of 0.1-3 say 0.22kilograms per square meter per hour (kmh). Typically, the units may showgood separation (measured in terms of w % organic oxygenate in thepermeate during pervaporation of an aqueous solution of organicoxygenate through a polyvinyl alcohol polyacrylic acid separating layeron a polysulfone support).

It will be apparent that the preferred membrane is one which gives goodseparation (i.e. low concentration of oxygenate in the permeate) andhigh Flux. It is a yields good Separation at a Flux which may be asgreat as ten times and commonly 5-6 times that attained when using e.g.a polyacrylonitrile support layer.

The Separation Factor S or Sep which represents the ability of themembrane to separate water is calculated as follows: ##EQU1## whereinX_(n) and X_(m) are the weight fractions of water and non-aqueouscomponents respectively in the permeate (P) and the feed (F). A systemshowing no separation at all would have a Separation Factor of 1; and asystem showing perfect 100% separation would have a Separation Factor ofinfinity. The process of the instant invention may have a SeparationFactor of as high as 1400--typically several hundred up to, say about300. Satisfactory operation may require a Separation Factor of at leastabout 1000 (this may vary substantially) although good commercialpractice may require Separation Factors which are higher. The process ofthis invention typically yields Separation Factors which aresatisfactory.

Practice of the process of this invention will be apparent to thoseskilled in the art from inspection of the following examples wherein, aselsewhere in this specification, all parts are parts by weight unlessotherwise stated. An asterisk (*) indicates a control example.

DESCRIPTION OF SPECIFIC EMBODIMENTS EXAMPLE I

In this example, which represents the best mode presently known ofcarrying out the process of this invention, the selective separatinglayer is mounted on a porous support layer of a commercially availablecomposite containing a non-woven polyester backing as carrier layer,bearing as porous support layer, a microporous membrane of a polyethersulfone which is free of isopropylidene moieties in the backbone chainand wherein the phenylene groups in the backbone are bonded only toether-oxygen atoms and to sulfur atoms. This polymer may becharacterized by molecular weight M_(n) of 25,000, water absorption @20°C. of 2.1 w %, glass transition temperature of 487° K., tensile strengthat yield of 12,200 psig at 20° C.; and coefficient of linear thermalexpansion of 5.5×10⁻⁵ mm/mm/°C. This polymer has a molecular weightcut-off of about 20,000 and has pore size of about 200A.

The separating layer is formed by mixing equal parts of weight of (i) a7 w % aqueous solution of polyvinyl alcohol PVA of molecular weightM_(n) of 115,000 and (ii) a 7 w % aqueous solution of polyacrylic acidPAA of molecular weight M_(n) of 250,000. The mix is spread on thesupport to form a film which is then cured at 150° C. for 15 minutes.

The membrane is evaluated in a pervaporation cell to which the charge isadmitted at 70° C. Permeate pressure is <5 mm·Hg at liquid nitrogentemperature.

In this preferred embodiment, the charge solution contains 95.1 w %isopropanol (IPA) 4.9 w % water. The permeate condenser contains anaqueous solution containing only 5.8 w % IPA. The Flux (kmh) is 0.22.The Separation Factor is 315.

EXAMPLE II

In this Control Example, the porous support layer is a polyacrylonitrilemembrane of thickness of about 50 microns having a molecular weightcut-off of about 40,000. The separating layer and the carrier layer arethe same as in Example I. Other conditions are as in Example I.

In Example II*, the charge contains 97.4 w % IPA and 2.6 w %water--which is comparable to the charge of Example I.

The Flux (kmh) attained in Example II* is only 0.04 kmh (at a permeateconcentration of 0.2 w % IPA).

From these Examples, it may be seen that use of the polysulfone membraneof this invention unexpectedly permits attainment of Flux which is(0.22/0.04 or) 5.5 times higher than is attained when using the controlpolyacrylonitrile membrane.

EXAMPLES III-XI

In this series of Examples, the procedure of Example I is carried outusing the membrane system of that example except:

(i) The separating membrane is 100% PVA/0% PAA in Example III*, 70%PVA/30% PAA in Example IV, and 50% PVA/50% PAA in all other Examples.

(ii) The PVA--containing membrane is cross-linked (at 125° C. for 15minutes) with glutaraldehyde in mole ratio (of glutaraldehyde to PVA) of0.2 (in Control Example III* and VII*) and of 0.4 (in Control ExampleVI*) and of 0.04 (in Control Example VII*).

(iii) Curing Temperature (°C) is 100° C. (in Example IX), 125° C. (inExamples III*, VI*, VIII*, and X), 150° C. (in Examples I, II, IV, andV), and 125° C.) (in Example XI)

                  TABLE                                                           ______________________________________                                               PVA/PAA   Feed Conc Perm Conc     Flux                                 Example                                                                              W Ratio   % IPA     % IPA   Sep   (Kmh)                                ______________________________________                                        III*   100/0     95.8      0.04    57,000                                                                              0.04                                 IV     70/30     95.2      29.8    46    0.09                                 V      50/50     95.8      7.1     298   0.19                                 VI*    50/50     96.7      6.5     422   0.01                                 VII*   50/50     96.7      0.5     5830  0.02                                 VIII*  50/50     96.7      3.0     947   0.06                                 IX*    50/50     95.1      87.4    3     2.6                                  X      50/50     95.1      45      24    0.70                                 I      50/50     95.1      5.8     315   0.22                                 XI     50/50     95.1      1.4     1370  0.14                                 II*    50/50     97.4      0.2     18,700                                                                              0.04                                 ______________________________________                                    

From the above Table, it is apparent that:

(i) Experimental Example I, which gives the highest Flux (0.22 kmh), atlow concentration of IPA in the permeate, is carried out using a 50/50PVA/PAV separating membrane on a polysulfone support--the separatingmembrane being cured at 150° C. with no external cross-linking;

(ii) Control Example II* which is generally comparable to ExperimentalExample I, (except that Example II* uses a polyacrylonitrile supportlayer whereas Example I uses a polysulfone support layer) gives Fluxwhich are only (0.04/0.22 or) 18% of those attained in Example I;

(iii) Control Example III*, VI*-VIII* which utilize "external"cross-linking with glutaraldehyde, yield undesirably low Flux(0.04-0.01-0.02-0.06) in contrast to e.g. Experimental Example I (0.22)which utilizes internal cross-linking;

(iv) Comparison of Experimental Examples IX, X, I, and XI show that asthe curing temperature increases over the 100° C.-175° C. range, theFlux drops and the concentration of IPA in the permeate also drops. Abalance between these two factors indicates that Example I shows bestpromise as a candidate for further consideration.

(v) Example IX* shows inter alia that a temperature of 100° C. (87.6 w %IPA in the permeate) is not high enough to cure the system properly.Curing should be done at 125° C. or above and preferably 150° C-225° C.,say 150° C.

EXAMPLES XII-XIX

In this series of Examples, the effect of time on membrane performanceis measured. In each case, the membrane is prepared as in Example Iexcept that the weight percent of PAA in the membranes is varied (theremainder being PVA) and the curing conditions are varied. TheSelectivity (w % IPA in the permeate) and the Flux (kmh) at 0 time andat greater than 48 hours are measured. Feed is 85 w % IPA at 70° C.

                  TABLE                                                           ______________________________________                                              Memb                Initial   Final                                     Ex-   W %     Curing      Performance                                                                             Performance                               ample PAA     T/t         Sel   Flux  Sel  Flux                               ______________________________________                                        XII   80      150° C./4 min                                                                      77    2.7   4.0  0.78                               XIII  60      "           54    2.1   5.2  0.83                               XIV   40      "           11    1.3   1.7  0.69                               XV    20      "           5.8   0.86  0.39 0.56                               XVl   80      150° C./10 min                                                                     64    2.6   2.5  0.84                               XVII  60      "           19    1.7   2.8  0.89                               XVIII 40      "           6.8   1.2   2.4  0.87                               XIX   20      "           1.3   0.6   0.37 0.50                               ______________________________________                                    

From the above Table, it is apparent that:

(i) Practice of this invention gives good Flux and Selectivity bothinitially and finally (after 48 hours);

(ii) Selectivity generally improves over time while Flux generally dropsover time;

(iii) Best initial Flux (of 2.7) is attained using the 80/20 membranecured at 150° C. for 4 minutes; and the Flux is high (0.78) at the endof the test;

(iv) Best Final Flux (0.89) is attained with 60/40 membrane cured at150° C. for 10 minutes.

It is a feature of this invention that the desired results are attainedby internal (or intermolecular) cross-linking by the reaction orinteraction of the PVA and the PAA and the polysulfone (PS) on/with eachother--as distinguished from the external cross-linking of the prior art(e.g. glutaraldehyde) cross-linking agents. It appears that the PAA (ofhigher molecular weight contributes acid functionality which aids thecross-linking with the PVA. The PAA apparently also retains unreactedcarboxylic acid functionalities which impart a hydrophilic character tothe final membrane. The product membrane is internally cross-linked; andthis contributes to its ability to dewater solutions at highertemperature which would normally dissolve PAA or PAA (q.v. theuncross-linked membrane of Nguyen loc cit.)

Although this invention has been illustrated by reference to specificembodiments, it will be apparent to those skilled in the art thatvarious charges and modifications may be made which clearly fall withinthe scope of the invention.

What is claimed:
 1. The method of separating a charge aqueouscomposition containing organic oxygenate selected from the groupconsisting of alcohols, glycols, and weak acids whichcomprisesmaintaining a non-porous membrane separating layer of a blendof a polyvinyl alcohol and a polyacrylic acid mounted on a polysulfoneporous support layer; maintaining a pressure drop across said non-porousmembrane separating layer; passing an aqueous charge compositioncontaining water and organic oxygenate selected from the groupconsisting of alcohols, glycols, and weak acids into contact with thehigh pressure side of said non-porous separating layer whereby at leasta portion of said water in said aqueous charge mixture and a lesserportion of organic oxygenate pass by pervaporation through saidnon-porous separating layer as a lean mixture containing more water andless organic oxygenate selected from the group consisting of alcohols,glycols, and weak acids than are present in said aqueous charge and saidcharge is converted to a rich liquid containing less water and moreorganic oxygenate selected from the group consisting of alcohols,glycols, and weak acids than are present in said aqueous charge;recovering from the low pressure side of said nonporous separating layersaid lean mixture containing more water and less organic oxygenateselected from the group consisting of alcohols, glycols, and weak acidsthan are present in said aqueous charge, said lean mixture beingrecovered in vapor phase at a pressure below the vapor pressure thereof;and recovering from the high pressure side of said non-porous separatinglayer said rich liquid containing a lower water content and more organicoxygenate selected from the group consisting of alcohols, glycols, andweak acids than are present in said charge.
 2. The method of separatinga charge aqueous composition as claimed in claim 1 wherein saidseparating layer is characterized by a PVA/PAA weight ratio of 0.1-0.5.3. The method of separating a charge aqueous composition as claimed inclaim 1 wherein said separating layer is characterized by a PVA/PAAweight ratio of 0.1-0.5.
 4. The method of separating a charge aqueouscomposition as claimed in claim 1 wherein said membrane is cured at 125°C.-225° C. for 1-30 minutes.
 5. The method of separating a chargeaqueous composition as claimed in claim 1 wherein said membrane is curedat 150° C.-225° C. for 1-30 minutes.
 6. The method of separating acharge aqueous composition as claimed in claim 1 wherein said chargeaqueous composition contains an alcohol.
 7. The method of separating acharge aqueous composition as claimed in claim 1 wherein said chargeaqueous composition contains isopropanol.
 8. The method of separating acharge aqueous composition containing organic oxygenate selected fromthe group consisting of alcohols, glycols, and weak acids whichcomprisesmaintaining a polysulfone porous support layer; maintaining, onsaid polysulfone porous support layer, a non-porous membrane separatinglayer of a blend of a polyvinyl alcohol and a polyacrylic acid in weightratio of polyvinyl alcohol to polyacrylic acid of 0.5-10, saidseparating layer having been cured at 125° C.-220° C. for 1-30 minutes;maintaining a pressure drop across said non-porous membrane separatinglayer; passing an aqueous charge composition containing water andorganic oxygenate selected from the group consisting of alcohols,glycols, and weak acids into contact with the high pressure side of saidnon-porous separating layer whereby at least a portion of said water insaid aqueous charge mixture and a lesser portion of organic oxygenatepass by pervaporation through said non-porous separating layer as a leanmixture containing more water and less organic oxygenated selected fromthe group consisting of alcohol, glycols, and weak acids than arepresent in said aqueous charge and said charge is converted to a richliquid containing less water and more organic oxygenate selected fromthe group consisting of alcohols, glycols, and weak acids than arepresent in said aqueous charge; recovering from the low pressure side ofsaid non-porous separating layer said lean mixture containing more waterand less organic oxygenate selected from the group consisting ofalcohols, glycols, and weak acids than are present in said aqueouscharge, said lean mixture being recovered in vapor phase at a pressurebelow the vapor pressure thereof; and recovering from the high pressureside of said non-porous separating layer said rich liquid containing alower water content and more organic oxygenate selected from the groupconsisting of alcohols, glycols, and weak acids than are present in saidcharge.
 9. The method of separating a charge aqueous compositioncontaining isopropanol as organic oxygenate which comprisesmaintaining apolysulfone porous support layer; maintaining a non-porous membraneseparating layer of a blend of a polyvinyl alcohol and a polyacrylicacid in weight ratio of polyvinyl alcohol to polyacrylic acid of 0.5-10,mounted on said polysulfone porous support layer, said separating layerhaving been cured at 125° C.-220° C. for 1-30 minutes; maintaining apressure drop across said non-porous membrane separating layer; passingan aqueous charge cmposition containing water and isopropanol as organicoxygenate into contact with the high pressure side of said non-porousseparating layer whereby at lest a portion of said water in said aqueouscharge mixture and a lesser portion of organic oxygenate pass bypervaporation through said non-porous separating layer as a lean mixturecontaining more water and less organic oxygenate than are present insaid aqueous charge and said charge is converted to a rich liquidcontaining less water and more organic oxygenate than are present insaid aqueous charge; recovering from the low pressure side of saidnon-porous separating layer said lean mixture containing more water andless organic oxygenate than are present in said aqueous charge, saidlean mixture being recovered in vapor phase at a pressure below thevapor pressure thereof; and recovering from the high pressure side ofsaid non-porous separating layer said rich liquid containing a lowerwater content and more organic oxygenate than are present in saidcharge.